The present invention relates to a hydraulic servo cylinder device for controlling a speed reduction ratio, clutch operation, etc., in a continuously variable speed transmission.
Various continuously variable speed transmission for continuously changing the speed of input rotation have heretofore been proposed for use on motor vehicles. For example, Japanese Laid-Open Patent Publication No. 56(1981)-95772 discloses a continuously variable speed transmission for use on a motor vehicle, inlcuding a closed circuit composed of a fixed-displacement hydraulic pump and variable-displacement hydraulic motor.
In such a continuously variable speed transmission, clutch control for starting or stopping the motor vehicle and speed reduction ratio control during running of the motor vehicle are carried out by a servo cylinder device based on the opening of the engine throttle valve, the vehicle speed, and other parameters. One well known type of such servo cylinder device comprises a cylinder with its interior divided into two cylinder chambers by a piston slidably fitted therein. A given line pressure is exerted in one of the cylinder chambers, whereas a hydraulic control pressure acts in the other cylinder chamber, the hydraulic control pressure being variably controlled to move the piston slidably in the cylinder chambers. According to one scheme, the piston is coupled to ratio varying means such as the swash plate of a swash-plate-type variable-displacement piston motor for controlling the speed reduction ratio dependent on the movement of the piston.
With the hydraulic servo cylinder device described above, when exerting the hydraulic control pressure in the other cylinder chamber while the line pressure is acting in one cylinder chamber, the piston does not move until the hydraulic control pressure builds up to a pressure level which counteracts the force that the piston receives from the line pressure. Therefore, if the hydraulic control pressure is initially zero, then the piston is operated with a response delay corresponding to a period of time required for the hydraulic control pressure in the other cylinder chamber to increase up to the above pressure level.